Standard sand blasting equipment consists of a pressure vessel or supply pot to hold particles of a blasting medium such as sand, a source of compressed air connected to the supply pot via a conveying hose and a means of metering the blasting medium from the supply pot, which operates at a pressure that is the same or slightly higher than the conveying hose pressure. The sand/compressed air mixture is transported to a nozzle where the sand particles are accelerated and directed toward a workpiece. Flow rates of the sand or other blast media are determined by the type of media and coating being removed. Commercially available sand blasting apparatus typically employ media flow rates of 10-20 pounds per minute. About 0.5 to 1 pound of sand are used typically with about 1.0 pound of air, thus yielding a ratio of 0.5 to 1.0.
When it is required to remove coatings such as paint or to clean relatively soft surfaces such as aluminum, magnesium, plastic composites and the like, or to avoid surface alteration of even hard materials such as stainless steel, less aggressive abrasives, including inorganic salts such as sodium chloride and sodium bicarbonate, can be used in place of sand in conventional sand blasting equipment. The media flow rate used for the less aggressive abrasives is substantially less than that used for sand, and has been determined to be from about 0.5 to about 10.0 pounds per minute, using similar equipment. The lower flow rates require a much lower media to air ratio, in the range of about 0.05 to 0.5.
However, difficulties are encountered in maintaining continuous flow of less aggressive abrasive media at the lower flow rates when conventional sand blasting equipment is employed. The fine particles of abrasive media such as sodium bicarbonate are difficult to convey by pneumatic systems by their very nature. Further, the bicarbonate media particles tend to agglomerate upon exposure to a moisture-containing atmosphere, as is typical of the compressed air used in sand blasting. Flow aids such as hydrophobic silica have been added to the bicarbonate in an effort to improve the flow, but maintaining a substantially uniform flow of bicarbonate material to the blast nozzle has been difficult to achieve. Non-uniform flow of the blast media leads to erratic performance, which in turn results in increased cleaning time and even to damage of somewhat delicate surfaces.
Commonly assigned U.S. Pat. Nos. 5,081,799 and 5,083,402 disclose a modification of conventional blasting apparatus by providing a separate source of line air to the supply pot through a pressure regulator to provide a greater pressure in the supply pot than is provided to the conveying hose. This differential pressure is maintained by an orifice having a predetermined area and situated between the supply pot and the conveying hose. The orifice provides an exit for the blast media and a relatively small quantity of air from the supply pot to the conveying hose, and ultimately to the nozzle and finally the workpiece. The differential air pressure, typically operating between 1.0 and 5.0 psi with an orifice having an appropriate area, yields acceptable media flow rates in a controlled manner. The entire contents of U.S. Pat. Nos. 5,081,799 and 5,083,402 are herein incorporated by reference.
A media metering and dispensing valve which meters and dispenses the abrasive from the supply pot through the orifice and to the conveying hose carrying the compressed air stream typically operates automatically whenever the compressed air is applied to the blast hose to begin the abrasive blasting operation. The media valve for use in the afore-mentioned metering and dispensing process as disclosed in U.S. Pat. Nos. 5,081,799 and 5,083,402 is characterized as a Thompson valve and is described in general in U.S. Pat. No. 3,476,440, the contents of which are herein incorporated by reference. The Thompson valve includes a metering valve stem which blocks the outlet of a discharge tube disposed between the supply pot and an air flow tube which is secured to and carries the compressed air to the conveying hose. When the blast nozzle is activated, the valve stem is lifted from the valve seat of the Thompson valve and allows a controlled amount of media to flow through the outlet of the discharge tube into the air flow tube. The valve as disclosed in U.S. Pat. No. 3,476,440 has been improved by placing the valve stem within a control sleeve which contains a plurality of orifices having different sizes, one of which can be placed in communication with the outlet of the discharge tube and the air flow tube by rotation of the media sleeve. When the valve stem is placed wholly within the control sleeve and closed, the orifice in the control sleeve is blocked such that media cannot flow from the discharge tube through the orifice in the media control sleeve and then into air flow tube. Upon operation of the blast nozzle, the valve stem is lifted through the sleeve and pulled away from the orifice to allow the media to flow from the pot to the discharge tube, through the orifice and into the air flow tube. The improved valve is described in commonly assigned U.S. Pat. No. 5,421,767, issued Jun. 6, 1995, and U.S. Pat. No. 5,401,205, issued Mar. 28, 1995, the contents of both of which are herein incorporated by reference.
As briefly discussed above, moisture is often added to the media in the supply pot during pressurization. Pressurization is provided from a supply of compressed gas (air) and pressure regulated to a piping T-connector which directs the compressed air through separate piping to the supply pot and the blast hose and nozzle. During pressurization of the supply pot, compressed air enters the media supply pot through a pop-up tube after the abrasive media has been fully loaded into the pot. Incoming air causes a pop-up valve slidably engaged in the pop-up tube to rise and seal off the media supply opening in the pot allowing pressurization of the pot and activation of the differential pressure media metering system described previously. Unfortunately, moisture accumulates in the air supply line to the supply pot and upon the initial pressurization of the media supply pot, the compressed air carries the collected pool of moisture up the pop-up tube and into the media pot moistening the media and causing portions of the particulate media to agglomerate. Still further, the compressed air itself may contain moisture in the form of fine droplets which are carried to the abrasive particles in the pot. The agglomerated media is not readily free-flowing which often causes a non-uniform media flow from the pot. The problem of moisture is exacerbated since the initial air expands rapidly causing the air to cool which consequently causes precipitation of the trapped moisture from the air onto the particulate media.
It would be worthwhile to provide a means to supply compressed air to the media supply pot for the differential pressure metering system which supply means would eliminate the problem of entrained moisture within the compressed air from leaving the pop-up tube and falling onto the particulate abrasive media in the supply pot.
In commonly assigned, copending application U.S. Ser. No. 161,528, filed Dec. 6, 1993, the substantial elimination of entrained moisture from precipitating onto the abrasive particles in the supply pot is achieved by providing a novel pop-up valve in the abrasive media supply pot. As disclosed therein the pop-up valve includes a pop-up valve stem which fits and is slidable within a pop-up valve tube which is secured to the compressed air supply tube. The pop-up valve tube includes an insert which prevents air and accumulated moisture from passing between the circumferential edge of the pop-up valve tube and the pop-up valve stem. Moisture which contacts the insert falls back into the compressed air supply line which can be periodically drained. The insert in the pop-up valve tube includes a central orifice which limits the expansion of the compressed air entering the pot to reduce cooling of the expanding gas and prevent precipitation of entrapped moisture. The entire contents of U.S. Ser. No. 161,528 is herein incorporated by reference.
Further, it would be most useful to prevent moisture present in the compressed air line from even entering the supply pot.